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Imagine you order a delivery of several glass vases in different colors. Each vase is sent as a separate parcel. What would you think of the courier if the parcels arrive apparently undamaged, yet when you open them, it turns out that all the redvases are intact and all the green ones are smashed to pieces? Physicists from the University of Warsaw and the Gdansk University of Technology have demonstrated that when quantum information is transmitted, nature can be as whimsical as this crazy delivery man.

Experiments on individual photons, conducted by physicists from the Faculty of Physics at the University of Warsaw (FUW) and the Faculty of Applied Physics and Mathematics at the Gdansk University of Technology (PG), have revealed yet anothercounterintuitive feature of the quantum world. When a quantum object is transmitted, its quantum property – whether it behaves as a wave or as a particle – appears to depend on other properties that at first glance have nothing to do with the transmission. These surprising results were published inthe research journal Nature Communications.

Wave-interferenceexperiments are some of the simplest and most elegant, and can be conducted byalmost anyone. When a laser beam is directed at a plate with two slits, weobserve a sequence of light and dark fringes. It has long been known that thefringes are visible even when just individual particles – single electrons orphotons – pass through the slits. Physicists assume that every individualparticle exhibits wave properties, passing through both slits at once andinterfering with itself.

The situation is verydifferent when it is possible to detect the path taken by a given photon orelectron and determine which slit the particle has passed through, at least inprinciple. When information about the particle path leaks from the system tothe observer, the interference disappears and instead of interference fringesno pattern is observed.

In order for photons toexhibit interference, their wavelengths must be the same, while electrons musthave the same energy. However, quantum particles have a number of otherproperties. For example, they can be polarized (their electrical field vibratesin a certain plane) or have different spin orientations (a quantum propertydescribing the dynamics of an object at rest).

"So far, it hasbeen generally assumed that additional properties such as spin and polarizationdo not have a non-trivial impact on interference. We decided to study the topicin more depth, and we were surprised by the results we obtained," saysProf. Konrad Banaszek (FUW).

The experiments byphysicists from the University of Warsaw and the GdańskUniversity of Technology started by generating heralded photons. "The namesounds complicated, but the idea is simple in itself," according to Prof.Czeslaw Radzewicz (FUW). "We generate photons using a process in whichthey must be created in pairs. When we register one photon, we can be certainthat the second was also born, and we know its properties such as direction orwavelength without destroying it. In other words, we use one photon to heraldthe generation of the second photon."

Each heralded photon wasdirected individually towards an interferometer, comprising two calcitecrystals. In the first crystal, the photon was split and then sent through botharms of the interferometer at the same time. In each arm, researchers alteredthe polarization of the photon (the plane of vibration of its electrical field)by introducing noise. In the second calcite crystal, the paths were recombinedto create a distinctive set of interference fringes, provided that the systemdid not leak any information as to which arm the given photon travelled along.The final stage of the experiment involved measuring the interference fringesusing silicon avalanche photodiodes.

"It turned out thatwe were able to use measurements of interference fringes to determine how muchinformation had leaked during transmission of the photon through theinterferometer. In other words, we could be certain whether any eavesdroppinghad taken place during transmission," says Dr Michal Karpinski (University of Warsaw,currently University of Oxford), responsiblefor building the experimental system and conducting the measurements.

The results haverevealed a new, surprising property of reality: the polarization of photons, orother internal degrees of freedom, play a highly non-trivial role ininterference between the two paths.

"It is almost asthough the quality of a courier delivery – for example, whether a glass vase deliveredinside a securely packed parcel is still in one piece – depends on whether thevase is green or red. In our world the color has no bearing on whether the vasearrives intact or not. However, the condition of the parcels our 'quantumcourier' delivers does indeed depend on internal properties that seem to havenothing to do with interference," according to Prof. Pawel Horodecki (PG).

The results allowphysicists to examine the fundamental properties of reality in new, morecomprehensive ways, as well as having practical applications in quantumcryptography. The Warsaw and Gdansk physicists have successfully derived ageneral inequality making it possible to precisely estimate the volume ofinformation leaking from the measurement system.

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The research was fundedby the European Union 7th Framework Programme and as part of the Foundation forPolish Science TEAM Programme, co-financed from EU funds – the EuropeanRegional Development Fund.

Physics and Astronomyfirst appeared at the University of Warsaw in 1816, underthe then Faculty of Philosophy. In 1825 the Astronomical Observatory wasestablished. Currently, the Faculty of Physics' Institutes include ExperimentalPhysics, Theoretical Physics, Geophysics, Department of Mathematical Methodsand an Astronomical Observatory. Research covers almost all areas of modernphysics, on scales from the quantum to the cosmological. The Faculty's researchand teaching staff includes ca. 200 university teachers, of which near 80 areemployees with the title of professor. The Faculty of Physics, University of Warsaw, is attended by ca. 1000 studentsand more than 140 doctoral students.

Even an individualphoton can travel along both arms of the interferometer at the same time. Whenit is unknown which path it is travelling along, we observe interference andthe appearance of interference fringes. A strong signal is visible where thecrests of light waves meet, and a weak signal is obtained at the meeting pointof the troughs. If it is possible to determine which arm the photon travelledalong, following leakage of information from the interferometer, the fringesdisappear. (Source: NLTK/Tentaris/Maciej Frolow)

June 23, 2011

The key to practical quantum computing and high-efficiency solar cells may lie in the messy green world outside the physics lab.

On the face of it, quantum effects and living organisms seem to occupy utterly different realms. The former are usually observed only on the nanometre scale, surrounded by hard vacuum, ultra-low temperatures and a tightly controlled laboratory environment. The latter inhabit a macroscopic world that is warm, messy and anything but controlled. A quantum phenomenon such as 'coherence', in which the wave patterns of every part of a system stay in step, wouldn't last a microsecond in the tumultuous realm of the cell.

Or so everyone thought. But discoveries in recent years suggest that nature knows a few tricks that physicists don't: coherent quantum processes may well be ubiquitous in the natural world.

and:

Robert Blankenship, a photosynthesis researcher at Washington University in St Louis, Missouri, and a co-author with Fleming on the C. tepidium paper, admits to some scepticism. "My sense is that there may well be a few cases, like the ones we know about already, where these effects are important," he says, "but that many, if not most, biological systems will not utilize quantum effects like these." But Scholes believes that there are grounds for optimism, given a suitably broad definition of quantum biology. "I do think there are other examples in biology where an understanding at the quantum-mechanical level will help us to appreciate more deeply how the process works," he says.

and:

Quantum coherence in photosynthesis seems to be beneficial to the organisms using it. But did their ability to exploit quantum effects evolve through natural selection? Or is quantum coherence just an accidental side effect of the way certain molecules are structured? "There is a lot of speculation about the evolutionary question, and a lot of misunderstanding," says Scholes, who is far from sure about the answer. "We cannot tell if this effect in photosynthesis is selected for, nor if there is the option not to use coherence to move the electronic energy. There are no data available at all even to address the question."